The present invention relates to variable gain amplifiers and, more particularly, to amplifiers having gains which are capable of being switched in discrete steps calibrated in dB. Although the present invention is described for use in measuring parameters in telephone transmission systems, it is to be understood that the principles described herein have utilization in any field in which it is desirable to calibrate amplifier gain in dB.
It is a common requirement for telephone system test equipment to have amplifiers with gains which are manually adjustable to particular gain levels calibrated in dB. Quite often the required gain range extends between negative gains (i.e. attenuation) and positive gain (i.e. amplification) values. For example, it is common to require a range of +10 dBm to -40 dBm for test level point (TLP) and transmit level (TXL) settings. It is desirable that the adjustments be in steps of 0.1 dB. In the prior art such adjustment is commonly realized with the use of three thumb wheel switches, as follows: (1) a first thumbwheel switch having two positions for polarity determination; (2) a second thumbwheel switch having four positions for tens unit determination (i.e. 0, 10, 20 and 30 dBm); and (3) a third thumbwheel switch having ten positions for units determination. It is clear that this arrangement permits settings between -39 dBm to +39 dBm, a large part of this range being outside either of the TLP and TXL setting requirements. In fact, the controlled amplifier is normally designed only to operate over the required range so that over much of the settable range of the switches the setting does not correspond to the amplifier gain. A major part of the difficulty lies in the fact that increments in settings when above zero dBm represent greater values whereas increments in settings when below zero dBm represent lesser values; the prior art did not accommodate this fact. Further, unduly complex and expensive circuitry is employed in prior art switching circuits to effect the desired setting capability.
It is therefore an object of the present invention to provide a simple, inexpensive amplifier circuit in which the gain is switchable from +K dB to -K dB, where K is some arbitrary number.
It is another object of the present invention to provide an amplifier with a gain which is switchable in calibrated steps of 0.1 dB, 1.0 dB, or 10 dB, over a range which includes both positive and negative gains.
It is a further object of the present invention to provide an extremely simple and inexpensive switching arrangement to achieve the aforementioned objects.